The invention relates to the manufacture of corrugated metal hose from thin-walled, seamless or butt-welded smooth tubing, and more particularly, to the formation of annular corrugations in such tubing on a continuous basis.
It has been known in the art, as disclosed in U.S. Pat. Nos. 3,128,821 and 3,353,389, that helical metal hose can be manufactured on a continuous basis by either rotating a tube through annular die rings which themselves are free to rotate, or by rotating the die rings about a non-rotating tube. Another process for making helical corrugations uses tapered helical dies rotating about a drawn tube moving in a straight line.
Annular metal hose is typically made by relatively slow procedures such as the internal bulging process and the external inward-forming process. The internal bulging process can produce excellent corrugation shapes, but operates at a relatively low speed and can only make relatively short lengths. The process utilizes a solid rubber bung which is compressed to prebulge the tube after which the corrugation is formed by axial compression. The external inward-forming method depends on either preforming the tube by mechanical pressure using multi-finger type dies, or by pregrooving using a rotary planetary motion around the tube, both prior to forming of the corrugation by axial compression of the tube.
Although helical metal hose has been able to be produced at a much faster rate, and thus at a lower cost than annular tubing, it has a tendency to twist when compressed or extended axially, leading to undesirable torsional stresses within the hose and at its fitting attachment joints. It is also considerably more difficult to assemble to end fittings since the weld must pass through the root of a corrugation. Alternatively, portions of the hose are left uncorrugated to facilitate the application of fittings. Because of the above factors, the industry has a substantial preference for annular metal hose.
Obviously, in view of its advantages, it would be advantageous if one could manufacture annular metal hose on a continuous fashion similar to the manner in which helical hose is formed. U.S. Pat. No. 2,429,491 discloses a forming tool having a plurality of split, helically mounted discs which can produce a relatively rigid, annularly finned, smooth bored tube of the type used in heat exchangers. However, the resulting tube, in which the fins are formed by displacing the metal of the tube wall, has no correspondence to a metal hose. In a metal hose, the wall thickness stays generally constant and the final hose length is much shorter than the original tube due to the fact that the wall is progressively formed inwardly and outwardly, without any significant change in wall thickness, as the corrugations are produced. U.S. Pat. No. 3,656,331 discloses an apparatus that purports to produce annular corrugated tubing with an annular die ring having an internal helical ridge of less than 360.degree. extent and a pitch equal to the desired corrugation pitch of the finished tube. U.S. Pat. No. 4,215,559 is related to U.S. Pat. No. 3,656,331, but provides for the die ridge to have a maximum height for more than 360.degree.. In U.S. Pat. No. 3,656,331, the depth of penetration is adjusted before the corrugation operation commences. During the operation, the tube is driven axially without rotation through a guide in a predetermined relationship to the speed of rotation of a die carrier about the axis of the tube.